In mobile telecommunications systems of the time division multiple access (TDMA) type, time division communication in the radio path takes place in successive TDMA frames each of which consists of several time slots. In each time slot, a short information packet is sent as a radio frequency burst of a finite duration, which burst consists of a number of modulated bits. For the most part, time slots are used for the transmission of control channels and traffic channels. On the traffic channels, speech and data are transmitted. On the control channels, signalling between a base station and mobile subscriber stations is carried out. The Pan-European mobile system GSM (Global System for Mobile Communications) is an example of a TDMA radio system.
For communication in conventional TDMA systems, each mobile station is assigned one time slot of traffic channel for data or speech transmission. Thus, a GSM system, for example, can have as many as eight simultaneous connections to different mobile stations on a same carrier frequency. The maximum data transfer rate on a traffic channel is restricted to a relatively low level according to the bandwidth in use as well as channel coding and error correction, for example in a GSM system to 9.6 kbit/s or 12 kbit/s. In addition, in a GSM system a half-speed traffic channel (max. 4.8 kbit/s) can be chosen for low speeds of speech coding. The half-speed traffic channel is established when a mobile station communicates in a specific time slot only in every second frame, in other words, in half-speed. A second mobile station communicates in every second frame in the same time slot. This is how the capacity of the system can be doubled as far as the number of subscribers is concerned, in other words, on the same carrier wave it is possible for up to 16 mobile stations to communicate simultaneously.
In the last few years, the need for high-speed data services in mobile communication networks has remarkably increased. Data transfer rates of at least 64 kbit/s would be needed to utilize, for example, ISDN (Integrated Services Digital Network) circuit switched digital data services. PSTN data services of the public telephone network, such as modems and telefax terminals of class G3, require faster transfer rates, such as 14.4 kbit/s. One of the growing areas of mobile data transfer requiring higher transfer rates is the mobile video service. As examples of this kind of services, security control by cameras and video databases can be mentioned. The minimum data transfer rate in video transfer can be, for example, 16 or 32 kbit/s.
The data transfer rates of the present mobile communication networks are not, however, sufficient to satisfy this kind of new needs.